This PPG will study the nature of interactions of two major body systems: neuronal circuits in the spinal cord and the musculoskeletal system. In Project I, the limits of pruning of regenerating motor axons to restore the innervation of muscular output elements of spinal circuits. We also will explore how, by enhancing axonal regeneration of modifying the process nerve injury. In Project II, the functional consequences of interactions between these two systems will be studied non-invasively in human subjects using dynamic MRI methods. The participation of output elements in the production of different functionally defined tasks will be compared in normal subjects and in patients with defined pathophysiologic deficits. In Projects III, the strategies used by spinal circuits to interact with the musculoskeletal system to produce movements will be studied using kinetic and kinematic analyses during ramp walking in cats. In Project IV, the nature and source of ongoing proprioceptive feedback during locomotor movements will be studied in self-reinervated cat muscles. These projects will receive support from four Cores. An Administrative Core will provide essential services to all of the Projects and other Cores. These services will include clerical tasks, grant accounting, personnel management, and human subject coordination. The administrative Core will also manage the activities of a Science Advisory Board. The Computer and Data Processing Core will provide hard hardware and programming support, statistical services, and data processing consultation to each of the projects. It will also provide local support to all computer workstations and to our local area network. The Animal Care and Use Core will provide pre-operative screening, post-operative monitoring, behavioral training, and kinesiologic assistance to all of the projects utilizing laboratory animals. The Machine and Electronics Shop Core will provide common design, fabrication, and maintenance for small devices used for electronic monitoring and process control, as well as larger devices such as walkways for conducting experiments. It is anticipated that the results of these projects will strengthen the science base for rehabilitation medicine through the elucidation of the ways in which spinal circuits and the musculoskeletal system interact, and how clinicians might use that new knowledge.